Photolithographic masks are used in lithography systems or for producing microstructured components, such as integrated circuits or LCDs (liquid crystal displays). In a lithography process or in a microlithography process, an exposure apparatus illuminates a photolithographic mask, a photomask or simply a mask. The light passing through the mask or the light reflected by the mask is projected, by using a projection microscope, onto a substrate (e.g. a wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection microscope in order to transfer the structure elements of the mask onto the light-sensitive coating of the substrate.
The positioning of pattern elements or structure elements on the surface of masks must be highly precise, i.e. deviations from the predetermined positions thereof or deviations from a critical dimension (CD) of a pattern element must lie in the nanometer range so as not to lead to errors on wafers during the exposure with the corresponding mask. The production of photomasks which can meet these requirements is very complex, susceptible to errors and hence expensive. Therefore, masks must be repaired wherever possible.
An important precondition for repairing defective masks is the finding and characterization of defects which are present, in particular of positioning defects or positioning errors (“registration errors” or simply “registration”). The detection of positioning defects and/or of the CD is complicated and difficult as these dimensions need to be established with an accuracy in the single-digit nanometer range, preferably in the sub-nanometer range.
Use is made of mask inspection microscopes or position determination apparatuses in order to examine positioning errors and/or the CD value. Two different groups of methods can be used for measuring structures, structure elements or pattern elements on a mask:
(a) Evaluation methods, which measure an image of the structure elements or of the pattern elements in absolute terms in respect of a reference point. Threshold-based methods are examples from this group. US 2012/0063666 A1 describes various embodiments of such analyses in detail. This group also includes centroid-based methods. A third example of this group are symmetry-correlation methods. The laid-open application DE 10 2010 047 051 A1 explains this type of evaluation on the basis of various exemplary embodiments.
(b) Evaluation methods which measure the image of structure elements relatively, i.e. which relate it to a reference image. This category includes methods which compare a measured image of structure elements with a reference image which was generated from design data of the pattern elements, i.e. which carry out a so-called “die-to-database evaluation”. In this case, a reference image is also referred to as a model reference image since it was generated from the model data of the design. The U.S. Pat. No. 8,694,929 B2 describes a method in which a reference image generated from design data is not simply compared to a measured image of a mask portion, but instead the mask parameters of the reference image and the optical parameters of the recording instrument, by means of which the measured image or the measurement image was recorded, are varied at the same time by simulation in order to determine the position difference between reference image and measurement image with high precision. US 2012/0121205 A1 describes a method which renders it possible to determine the position of a second structure element relative to a first structure element or a reference element (“die-to-die evaluation”).
The evaluation methods mentioned in (a) are advantageous in that they supply an absolute position specification for the structure or pattern elements of a mask lying in an image field. Here, an image field shows a part or a portion of a mask. However, these methods are disadvantageous in that they also measure artefacts of an optical position determination apparatus (e.g. aberrations, apodization, optical proximity effects, and image field distortions) and hence supply a measurement result which is falsified by the properties of the position determination apparatus.
The methods mentioned in (b) have the advantage that they are largely independent of the optical aberrations of the position determination apparatus in the case of a suitable application. If a model reference image is used, i.e. if die-to-database evaluation is carried out, this is dependent on the quality of the model function. A further advantage of the methods based on a reference image lies in the higher tolerance thereof in relation to pattern elements which have a poor signal-to-noise ratio (SNR). This renders the methods specified in (b) advantageous, for example for small structure elements in the vicinity of the resolution limit of the position determination apparatus. In contrast to the image evaluation methods listed in (a), the entire image content or all pixels are used for the comparison of the measured image and reference image in the methods based on a reference image.
On the other hand, what is disadvantageous in the case of the methods based on a reference image is that the result of the position determination of pattern elements is only known relative to the reference image. This is particularly relevant in the case of methods which are based on a measured reference image (“die-to-die”) since the reference image in this case may itself have an unknown position and/or positioning error. Furthermore, the position of a measurement table which carries the mask during the recording may be afflicted by errors. Moreover, the settings of the position determination apparatus may not be ideal when recording a reference image.
The present invention is therefore based on the object of specifying a method and an apparatus for determining a position of a structure element of a photolithographic mask, which avoid the aforementioned disadvantages of the prior art at least in part.